Enkephalin-like peptides immunoreactive to (met5) or leu5)-enkaphalin are contained in adrenal chromaffine cells and in splanchnic nerve plasma obtained from dogs with an indwelling catheter in the adrenal lumbar vein contains a higher content of enkephaline-like peptides than plasma from jugular or femoral vein. Electrical stimulation of the splanchnic nerve increase the enkephaline-like peptide content in adrenal venous plasma. Injection of morphine also increases the plasma content of these peptides. Pretreatment of dogs with naloxone or prior splanchnic nerve transection prevents this increase.